The present invention relates to a method of producing a superconducting wire including an oxide superconductor. The superconducting wire may be used for magnet coils of, for example, the nuclear magnetic resonance imaging apparatus and the particle accelerator. In this specification and claims, the term "superconducting wire" means a superconducting wire, cable, braid, tape and a superconducting piece of a similar configuration. Some of the Inventors proposed oxide superconductors in the following applications as joint inventors: U.S. patent application Ser. No. 7/170,019 filed Mar. 18, 1988 entitled "Method of producing a superconducting wire using alloy preform", now U.S. Pat. No. 4,885,273, U.S. patent application Ser. No. 7/170,018 filed on Mar. 18, 1988 entitled "Method of producing oxide superconducting wire and oxide superconducting wire produced by this method"; U.S. patent application Ser. No. 7/191,436 filed on May 9, 1988 entitled "Superconductive electric wire and method for making it"; and U.S. patent application Ser. No. 07/178,223 filed on Apr. 6, 1988 entitled "Oxide superconductor and manufacturing method thereof" The disclosure of each of these four U.S. patent applications is incorporated herein by reference.
Recently, various oxide superconductors have been discovered which show very high values of the critical temperatures (Tc) at which the transition from a normal conductive state to a superconductive state takes place. Since such an oxide superconductor shows much higher critical temperature than conventional alloy or intermetallic compound superconductors, it is considered that oxide superconductors will highly promise for practical superconducting materials.
For producing cables having such an oxide superconductor, the following typical production methods have been proposed. In one method, material powders of the oxide superconductor were mixed at a predetermined ratio to prepare a mixed powder, which is then charged into a metallic pipe, which is in turn reduced in diameter and then heat treated to produce the superconductor. In another method, the mixed powder is compression molded to form a green compact, which is heat treated to produce a superconducting sinter. The sinter is pulverized into a powder, which is charged into a metallic sheath, which is reduced in diameter together with the charged powder and is then subjected to heat treatment to produce a superconducting wire.
These methods are disadvantageous in that it is difficult to produce long superconducting wires or cables with excellent superconductivity. The reason is that it is rather hard to completely uniformly mix superconductor material powders with the result in that it is difficult to obtain a homogeneous crystal structure of the superconductor along the entire length of the cable. Another disadvantage is that it is rather difficult to control the composition of superconductors. Still another disadvantage is that the oxide superconductor is a ceramic and is hence inferior in mechanical strength to other conventional superconductors.
Accordingly, it is an object of the present invention to provide a method of producing a superconducting wire including an oxide superconductor, in which method the superconductor is easily controlled to have a predetermined composition with both excellent superconductivity and high mechanical strength.